Digital image analysis of two-dimensional Na,K-ATPase crystals: dissimilarity between pump units.

Two-dimensional crystals of purified Na,K-ATPase were induced by treatment with phospholipase-A2 and vanadate. The negatively stained crystals were imaged by electron microscopy and analysed by digital image processing. Two-dimensional averaged projections of the crystals were calculated by the technique of correlation analysis, utilizing SPIDER (System for Processing of Image Data in Electron microscopy and Related fields) image processing software. The calculated dimensions of the unit cell were found to be 13.3 X 4.59 nm with included angle of 98 degrees, comparable to those reported by others. However, the two protomers of the unit cell were found always to be dissimilar in shape and in orientation. All protomers of one side of the dimer ribbon had a triangular outline, and all protomers of the opposing side had a comma shape. This dissimilarity could be explained by two orientations of identical protomers: one orientation for one side of the dimer ribbon, and another orientation for the protomers of the opposing side of the ribbon. An alternative explanation is that the protomers of one side of the dimer ribbon are actually in a conformation different from that of the protomers of the opposing of the ribbon.